1. Field of the Invention
The present invention relates to a method for producing titanium alloy, and more particularly to a method for producing titanium alloy by reducing titanium tetrachloride and alloy component metals or their chlorides with a reducing metal agent.
2. Description of the Prior Art
A conventional method for producing titanium alloy generally comprises the steps of preparing a consumable electrode, obtained by mixing sponge titanium particles with alloy component elements, forming the mixture into a block, and melting the consumable electrode in a vacuum arc melting furnace to prepare an ingot of titanium alloy. For example, Ti-6Al-4V alloy is produced by crushing sponge titanium obtained by the Kroll process into particles and mixing this with Al-V alloy and Al in pellet form. The mixture is subjected to press to form briquettes which are in turn welded in an inert atmosphere to form an electrode to be melted in the vacuum arc melting furnace to obtain an ingot of the alloy. This conventional consumable electrode type arc melting method therefore cannot continuously produce the alloy because a consumable electrode must be prepared by mixing sponge titanium with alloy component elements and forming the mixture into a compact shape. Thus, the arc melting method requires at least four steps, namely, a reduction step, a separation step, a crushing and mixing step and a melting step, which are all quite independent, in the production of a metallic titanium ingot.
Thus, the production of titanium alloy by the consumable electrode type arc melting method generally has the following disadvantages.
It cannot operate continuously from the production of titanium sponge by metal halide reduction to the melting of metallic titanium, resulting in several complicated separate steps being required to produce the titanium alloy.
The titanium sponge produced in the reduction step tends to be contaminated with the material of the reaction vessel because it is very reactive.
The titanium sponge tends to be contaminated with moisture and air in the separation and purification step.
Furthermore, there is considerable increase in equipment, energy and labour costs.
Another method for the production of titanium alloy by the reduction of a metal halide with a reducing metal agent is known in which the reaction temperature is maintained above the melting point of the titanium alloy to be produced by addition of alloy component metals or their chlorides, resulting in the titanium alloy being produced in a molten states. The molten titanium alloy product is then removed as a melt.
Alternatively, the product can be cooled to solidification in a reducing vessel and then continuously drawn out in the form of a ingot from the vessel.
As an example, Japanese Patent Application Laid-Open publication No. 35733/1981 discloses a method for producing an alloy by reacting a halide of a metal reactant particularly its chloride, with a reducing agent at a temperature above the melting point of the alloy to be produced. The alloy is solidified as it is drawn out from the reaction zone where reduction takes place. The alloy is kept in a liquid state within the reaction zone at a temperature above the boiling or sublimation point of the other reaction products at the pressure for the reduction, while the other reaction products are kept in gas form and substantially continuously drawn out the reaction zone.
Also, Japanese Patent Publication No. 19761/1971 discloses a method for producing an alloy by reducing a metal halide with a reducing metal at a temperature above a melting point of the alloy to be produced. In this case, the reduction takes place in a closed reaction zone in a reaction vessel at a pressure which is at least equal to the vapour pressure of the halide of the reducing metal.
Various other similar methods have attempted to solve the problems of the consumable electrode type arc melting method by keeping the reaction temperature above the melting point of the alloy to be produced during reduction of a metal halide with a reducing metal agent. Although these methods are disclosed in the patent literature, they have not been commercialized on an industrial scale.
The main reason is believed to be the difficulty in selecting a material for the reaction vessel which can withstand a sufficiently high temperature to keep high melting point reactive metals such as titanium, zirconium, or the like in a molten state.
The above-described method disclosed in Japanese Patent Publication No. 19761/1971 is to reduce chlorides of alloy component metals including titanium tetrachloride with magnesium to produce titanium alloy in a reaction zone while keeping the temperature of the reaction zone above the melting point of the titanium alloy and the pressure of the reaction zone above the partial pressure of the magnesium chloride by-product at that temperature. Unfortunately, under these conditions, the magnesium boils resulting in a failure to provide sufficient magnesium to reduce completely the chlorides of the alloy component metals, including titanium tetrachloride, in the reaction zone. This often leads to the production of titanium subchlorides such as titanium trichloride and titanium dichloride.
Also, in this method, the reactants, including titanium tetrachloride and magnesium, are supplied through graphite pipes to a molten layer of the reaction product at the bottom of the reaction vessel, so that the reaction takes place in the molten layer. This causes the ends of the graphite pipes to be corroded by the active molten titanium alloy product. In addition, the molten titanium alloy product contacts the reactants at a relatively low temperature at the open end of the pipes, solidifying the reactants and so clogging the pipes. Furthermore, since the reaction takes place in the molten layer of titanium alloy, the titanium alloy product is contaminated with unreacted reactants, by-products and the like. Finally, insufficient magnesium in the reaction zone leads to a decrease in reaction efficiency.